A Bourdon tube pressure gauge is widely used in order to measure pressures of various kinds of gases. The Bourdon tube pressure gauge includes a metal pipe having a flat cross-section and having an entire shape formed in a C type, a spiral type, a helical type, or the like, is obtained by applying a fact that a curvature of the metal pipe is changed in accordance with internal pressure change, and characterized in having a structure relatively simple, and the one having a measurement range up to approximately 250 MPa is commercially available.
The Bourdon tube pressure gauge generally has a mechanism in which displacement of a tip of a Bourdon tube caused by pressure change is transmitted to a rotary piece, and the pointer is rotated by a gear provided at the rotary piece and a gear provided at an outer periphery of a shaft of the pointer.
In the Bourdon tube pressure gauge having such a mechanism, since the pointer is operated by the gears, and the pointer swings due to rattle at an engagement portion of the gears, thereby causing a problem in which pressure indication becomes unstable. Furthermore, when there is rapid pressure change, there may be a problem in which the gears are worn out by hard friction between the gears.
To solve such a problem, Patent Literature 1 and Patent Literature 2 disclose techniques in which a pointer is biased in one rotary direction by attaching a spiral spring to a rotary shaft thereof. However, since displacement of a Bourdon tube is transmitted by the gears in the above-described Bourdon tube pressure gauges, there is still room for improvement in terms of wear.
As a countermeasure therefor, the applicant of the present application discloses a technique in Patent Literature 3 in which displacement of a Bourdon tube can be transmitted by magnetic force. FIG. 8 is a view illustrating a Bourdon tube pressure gauge disclosed in Patent Literature 3. In the drawing, a Bourdon tube 12 is a tube having a substantially elliptical cross-sectional shape, and having a tip thereof closed by bending the tube in a manner drawing an arc.
A substantially center of a rotary piece 13 is press-fitted to a rotary piece shaft 20, and one side thereof is connected to a tip of the Bourdon tube 12 via a rod 14. Additionally, the rotary piece 13 has the other side formed in a fan-shaped portion 13c including an arc-shaped edge centering the rotary piece shaft 20, and an arc-shaped permanent magnet 21 is fixed on an outer peripheral surface side of the fan-shaped portion 13c. The permanent magnet 21 has multiple N poles and S poles alternately arranged in a circumferential direction on a circumference thereof. The rotary piece shaft 20 is rotatably supported by a base plate not illustrated.
A pointer 17 is press-fitted and held by a pointer shaft 18. The pointer shaft 18 is rotatably supported by the base plate not illustrated. A cylindrical permanent magnet 19 is fixed to the pointer shaft 18 in the circumferential direction so as to face the permanent magnet 21 of the rotary piece 13. The permanent magnet 19 has multiple N poles and S poles are alternately arranged. Rotation of the permanent magnet 21 is transmitted to the permanent magnet 19 by magnetic coupling between the N poles and the S poles alternately arranged on the circumferential surface in each of the two permanent magnets 19 and 21.
Here, when a pressure P is applied to the Bourdon tube 12 as indicated by an arrow A, the Bourdon tube 12 is elastically deformed, and a tip thereof is displaced upward in FIG. 8. Due to displacement of the Bourdon tube 12, a right portion of the rotary piece 13 is displaced upward via the rod 14, and the arc-shaped permanent magnet 21 fixed to the rotary piece 13 is rotated. When the permanent magnet 21 is rotated, the permanent magnet 19 fixed to the pointer shaft 18 is synchronously rotated in accordance with movement of the permanent magnet 21, and the pressure P is pointed by the pointer 17 held by the pointer shaft 18.
However, in the above-described pressure gauge in the related art, there may be problems that indication accuracy of the pointer 17 is not stabilized because the pointer shaft 18 is attracted in a direction approaching the rotary piece shaft 20 by attraction force between the permanent magnets 19 and 21 respectively fixed to the rotary piece 13 and the pointer shaft 18, frictional force between the pointer shaft 18 and a member supporting the pointer shaft 18 is increased, and the frictional force acts as a load.
Additionally, when rapid pressure change is instantaneously applied to the Bourdon tube 12, there may be a problem in which the pointer 17 cannot indicate a correct pressure P because the Bourdon tube 12 is suddenly elastically deformed by a large amount, the rotary piece 13 is also displaced due to this deformation, magnetic coupling (synchronization) at a proper position between the permanent magnets 19 and 21 respectively fixed to the rotary piece 13 and the pointer shaft 18 is eliminated, and the permanent magnet 19 fixed to the pointer shaft 18 is deviated in a rotary direction, thereby causing magnetic coupling at an improper position (loss of synchronism).